The invention relates to a device for applying electromagnetic microwave radiation in a plasma inside a hollow glass substrate tube. The device generally includes an annular space enclosed by a metal wall, and further includes an input port and an applicator slit, the applicator slit being an output port acting as a radial waveguide, wherein, at the input port, a first end of an elongated microwave guide is attached. The microwave guide is in communication with microwave generating means via a second end of the microwave guide.
The invention further relates to a method for manufacturing an optical fibre preform using the device for applying electromagnetic microwave radiation, and an optical fibre obtained from the optical fibre preform.
A device for applying electromagnetic microwave radiation in a plasma cavity is disclosed in U.S. Pat. No. 7,650,853, which is from the same inventors. U.S. Pat. No. 7,650,853 relates to the provision of a microwave applicator by means of which a stable plasma having favorable geometric properties can be generated and maintained, which microwave radiation has only one electromagnetic field distribution at least in one direction perpendicular to the propagation direction.
European Patent 1867 610 in the name of the present inventors discloses an apparatus for carrying out a Plasma Chemical Vapour Deposition (PCVD) deposition process, wherein in a coaxial waveguide an antenna is movable, wherein the antenna bisects a feed waveguide and a guide element is present in the interior of the feed waveguide.
U.S. Pat. No. 5,223,308 discloses a deposition apparatus that includes a microwave generator, wherein a rectangular microwave waveguide is used for generating an electromagnetic field of intense microwave energy in a space in which an elongated hollow tube is moved continuously. Such a tube is formed of a synthetic resin, for example a nylon material, wherein a coating of silicon oxide, silicon nitride or silicon oxycarbide is applied by means of the aforesaid apparatus, which tubes are used as pipes for the hydraulic air conditioning system in automobiles in order to minimize the loss of liquid coolant, for example freon, into the atmosphere.
U.S. 2003/0104139 relates to an apparatus for depositing a PCVD coating on the interior of a hollow glass tube, wherein an applicator includes a waveguide and an applicator head is used, wherein the waveguide functions to guide microwaves from a microwave generator to the applicator head. The waveguide has an elongated axis and a rectangular cross-section having a long axis and a short axis arranged perpendicular to the elongated axis of the waveguide. A glass tube is positioned within the applicator head, and the applicator head is moved over the hollow glass tube, where a coating is to be deposited, along the longitudinal axis of the tube.
U.S. 2003/0115909 relates to an apparatus for depositing one or more glass layers on the interior of a hollow substrate tube, wherein an activator space of a microwave applicator surrounds the hollow substrate tube, with the microwaves generating a plasma in the interior of the hollow substrate tube causing the glass-forming precursors to deposit SiO2 onto the interior of the substrate tube.
U.S. 2004/0045508 relates to a plasma-activated CVD system in which a cooler for cooling the annular waveguide is provided, whereby it is intended to protect the annular waveguide from heat and attain a safe and stable supply of the microwave. The cooler is disposed between the annular waveguide and the quartz tube as the reaction chamber, i.e. a cooling pipe for passing therethrough a refrigerant such as water, oil, or gas is mounted to an inner surface of the inner periphery wall of the annular waveguide, thereby constituting the cooler. The cooling pipe is embedded in the inner periphery wall of the annular waveguide.
U.S. Pat. No. 4,125,389 relates to a method of manufacturing an optical fibre by plasma activated deposition in a tube, and to an apparatus for such a method, in which a substrate tube of fused silica is placed through a microwave cavity connected to a high power microwave generator by a waveguide. Gas is fed through the tube and cooling gas is passed along the outside of the tube, in the microwave cavity, as a cooling gas such as nitrogen is passed between the metal cavity and the outside wall of the tube.
U.S. Pat. No. 4,844,007 relates to a device for providing glass layers on the inside of a tube including a gas supply device that is connected to one end of the tube, a furnace for heating the tube, a resonator including a resonant cavity for generating a plasma in the tube, and means to move the resonator and the tube relative to each other, a high-frequency generator connected to the resonant cavity and a vacuum pump connected to the opposite end of the glass tube. The resonator includes a cooled body having a duct for receiving the tube, of which body at least the duct wall consists of a readily heat-conducting metal and in which the wall of the duct facing the tube is provided with a heat-insulating layer. The cooling capacity of the resonator is reduced by the presence of the heat-insulating layer and the resonator is cooled via cooling water ducts.
An apparatus for manufacturing optical fibres is disclosed in U.S. Pat. No. 6,849,307, which is from the same inventors. The apparatus may be used within the context of manufacturing from which an optical fibre can be drawn. According to the method for manufacturing such a preform, an elongated vitreous substrate tube, made of quartz for example, is coated on its interior cylindrical surface with layers of doped silica, for example germanium-doped silica. This can be achieved by positioning the substrate tube along the cylindrical axis of a reaction zone and flushing the interior of the tube with a gaseous mixture, for example O2, SiCl4 and possible dopants, e.g. GeCl4. Localized plasma is concurrently generated within a cavity, causing the reaction of Si, O and Ge so as to effect direct deposition of Ge-doped SiO2 on the interior surface of the substrate tube. Since such deposition only occurs in the vicinity of the localized plasma, the reaction zone must be swept along the cylindrical axis of the tube in order to uniformly coat the whole length of the tube. When coating is complete, the tube is thermally collapsed into a massive solid rod having a Ge-doped silica core portion and a surrounding undoped silica cladding portion. If an extremity of the rod is heated so that it becomes molten, a thin glass fibre can be drawn from the rod and wound on a reel, with the fibre having a core and cladding portion corresponding to those of the rod. Because the Ge-doped core has a higher refractive index than the undoped cladding, the fibre can function as a waveguide for optical signals, for example for use in propagating optical telecommunication signals. The gaseous mixture flushed through the hollow glass substrate tube may contain other components and dopants, for example C2F6 causes a reduction in the refractive index of the doped silica. The solid preform may be placed in a “jacket tube” made of undoped silica prior to the drawing procedure in order to increase the quantity of undoped silica relative to doped silica in the final fibre. Another possibility is to apply an extra amount of silica to the “overcladding” by means of a plasma process or outside vapour deposition (OVD) process.
The use of such an optical fibre for telecommunication purposes requires that the optical fibre be substantially free from defects such as discrepancies in the percentage of dopants, undesirable cross-sectional ellipticity, and the like, because, when considered over a large length of the optical fibre, such defects may cause a significant attenuation of the signal being transported. It is important, therefore, to realize a very uniform and reproducible PCVD process because the quality of the deposited PCVD layers ultimately determines the quality of the optical fibres. Thus, it is important that the plasma generated in the resonant cavity be rotationally symmetrical around the cylindrical axis of the cavity. On the other hand, the costs of the production process will be advantageously affected if the preform can be given a larger diameter, since larger fibre lengths can then be obtained from a single preform. Increasing the diameter of the resonant cavity enables the use of substrate tubes having increased diameters, however leads to a plasma having a deteriorated rotational symmetry, and such can only be generated by using much higher microwave power.
In the aforesaid PCVD apparatus, energy from a device capable of generating microwaves, for example a microwave oven, must be transferred to the annular resonant cavity so as to form a plasma zone in the interior of the substrate tube. This means that the microwaves are supplied to the feed waveguide and can subsequently reach the annular resonant cavity via a waveguide.
The present inventors have found that using high doped hollow substrate tubes may cause problems during the inside deposition process of the plasma type. Examples of problems that may occur are related to melting of the substrate tube, or a partial premature collapse of the substrate tube during the inside deposition phase. These problems require an early termination of the deposition process, and are highly undesirable form the viewpoint of process stability and safety. While one solution to these problems is a lower furnace temperature, lowering the furnace temperature can disadvantageously lead to the deposition of lower quality glass layers.